Defining Areas of the Network

In both fixed and mobile networks, it is possible to divide the network into various
parts according to their function, some parts of the
network deal with connection to the user and others handle the management of the
users and the services they wish to consume.

The Access Network
In the fixed network, the access network consists of the wires and
cable that allow users to be connected to the central part of the network where
access to services can be obtained. This is often the largest and most complicated
part because it relies on a pair of copper wires or cable being delivered to every subscriber
in the network. In a very large network, this can mean that tens of millions
of wires must be connected from the users’ premises and run back toward the core
network. To reduce the overall complexity, the access network will often contain
devices that will allow the information from many wired connections to be sent on
a single cable. This reduces the total number of wires in the system.
In some parts of the world, the copper infrastructure is in very poor condition
due to lack of maintenance or local conditions. Some operators choose to establish
wireless local loop (WLL) style systems, which eliminate traditional copper wires.
WLL systems should not be confused with mobile radio systems because they form
part of the connection to a user’s fixed device.

The access network is able to carry voice information (most likely in analog format)
and data where the condition of the line allows. It is becoming more common
for domestic premises to have both a speech and a broadband connection with the
network service provider.

The Radio Access Network
The difference between radio system and fixed system is that the user’s terminal
is a mobile device. The user then has the freedom to move anywhere within the
system radio coverage area and expect to communicate with the core network.
Radio access networks (Figure 1.15) are most often associated with cellular phones
systems, but PMR and other radio systems such as pagers can be considered mobile
radio systems.

The purpose of the radio access network is to allow the users to make connections
to the core network anywhere in the coverage area of the network. It is able to
do this by placing radio base stations around the area where service is required. The
mobile device then uses radio signals to relay the user’s information or voice to the
core network via the base station.
Satellites can also be part of a radio access network. The satellites are simply
radio base stations that orbit the Earth instead of being fixed on the ground.

Core Networks
If the radio access network is there to provide connection to the core network, then
the core network exists to provide the service to the user.
The core network (Figure 1.16) is a complex system of switches, interfaces,
databases, and transmission systems, all designed to offer the users of the network
telephone call, a relatively simple service for the user, but one that still requires a
great deal of complexity in the networks.

Today’s modern systems often separate the core of the network
into two distinct areas: (1) voice service and (2) data services. The core network
concerned with voice primarily deals with the routing of telephone calls and those
services normally associated with telephony, such as call waiting, call hold, etc.
Connections to external telephone systems such as the other public telephone systems
or between mobile networks or the international networks are also made from
the voice-based core network. The data side of the network deals with data connections
and the routing of data to connected data terminals. External connections are
also supported, most probably to the public Internet or to local networks associated
with business systems.


Other Types of Networks
There are some other types of networks that perhaps do not fit into the precise
definitions given above. These are networks that perhaps do not have an access
network and do not offer their services directly to members of the public. Their customer
is more likely a large business or the fixed and mobile operators themselves.
These networks provide long-distance connections within countries or connections
between countries. They are the national and international carriers.
It is often the case that a large incumbent fixed operator will offer its service to
domestic subscribers via an access network and also provide long-distance connections
via its core networks for enterprises requiring fixed point-to-point connections.
These types of connections are often referred to as leased lines.
Some of these very large (Tier 1) networks will also own or connect to the international
networks to allow calls to be routed between countries around the world.